Britain's leading authority on voting methods believes that moving to a proportional representation system is now "inevitable" and that the "transition" to reform has already begun.

In a paper to be published next month, Professor Patrick Dunleavy of the London School of Economics argues that it is only a matter of time until Labour switches to PR to bolster its declining position. The analysis by Britain's top academic authority is a boost to The Independent's Campaign for Democracy which has so far attracted more than 35,000 responses from readers.

By yesterday 20,645 people had filled in forms calling on Tony Blair to introduce a more proportional system so "the result of a general election is more representative of their wishes". A further 14,704 have signed a petition on The Independent's website calling for reform.

Professor Dunleavy's analysis of voting patterns found that the establishment of a "multi-party" system - which traditionally precedes the move from a first-past-the-post voting system to PR - "has already happened" in Britain.

The analysis of voting in the UK, has found that on average five parties - such as the Greens, Scottish or Welsh Nationalists and UKIP - stand in elections and attract a significant share of the vote.

Evidence from other countries which have switched to PR show that the establishment of a multi-party system is the precursor to electoral reform. Government or "incumbent elites" delay switching to PR until they "are forced to do so by the electorate".

He warns that if the main political parties continue to resist the introduction of PR then "the risk of incumbents losing elections catastrophically will grow".

It only a matter of time before incumbent elites - currently the Labour Party - to "decide that it is in their own interest to avoid the risk of losing out completely and instead try to stabilise their vote share and legislative representation by introducing a PR electoral system."

Professor Dunleavy, professor of political science, who helped to frame the voting system for the London mayoral elections and advised the Jenkins review on PR, says that "some form of transition of representation at Westminster seems inevitable as existing multi-party politics develops further."

Professor Dunleavy's analysis, published in next month's Parliamentary Affairs journal, draws on research showing that, where the shift to PR has occurred abroad, "the effective number of parties grows before they make the change, and does not subsequently grow further after PR."

In the UK there is now a "long running trend" to vote for a wide range of parties in Britain, reducing the share of the vote for the Tories and Labour.

Organisations in favour of electoral reform urged Tony Blair to embrace PR before the Labour Government is forced into it.

"First past the post worked reasonably well when it was a two-party culture in Britain, but now we are a multi-party democracy it produces results that are skewed and unrepresentative of people's views," said Nina Temple, the director of Make Votes Count.

"It's time that the Government stopped living in denial and embraced reform as a positive means of reconnecting politics to the public."

Labour gained a 67-seat majority in the Commons at the general election last month with just over 35 per cent of the vote and the support of only 22 per cent of the electorate.